Integrated Silicon profilometer and AFM head

ABSTRACT

A topographic head for profilometry and AFM supports a central paddle by coaxial torsion bars projecting inward from an outer frame. A tip projects from the paddle distal from the bars. The torsion bars include an integrated paddle rotation sensor. A XY stage may carry the topographic head for X and Y axis translation. The XYZ stage&#39;s fixed outer base is coupled to an X-axis stage via a plurality of flexures. The X-axis stage is coupled to a Y-axis stage also via a plurality of flexures. One of each set of flexures includes a shear stress sensor. A Z-axis stage may also be included to provide an integrated XYZ scanning stage. The topographic head&#39;s frame, bars and paddle, and the XYZ stage&#39;s stage-base, X-axis, Y-axis and Z-axis stages, and flexures are respectively monolithically fabricated by micromachining from a semiconductor wafer.

CLAIM OF PROVISIONAL APPLICATION RIGHTS

This application claims the benefit of U.S. Provisional patentapplication No. 60/008,495 filed on Dec. 11, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to that area of metrologictechnology concerned with measuring a surface's topology, and, moreparticularly, to profilometry, and to atomic force microscopy ("AFM),also sometimes referred to as scanning force microscopy ("SFM").

2. Description of the Prior Art

Recently, the field of surface profilometry has expanded greatly. Inaddition to advances in classical profilometry, the nascent fields oftunneling force microscopy and AFM have greatly enlarged the interest,scope and capability of profilometric technology.

Classical profilometry scans a surface along orthogonal X-axis andY-axis directions using a diamond tipped stylus while measuring thestylus' vertical (Z-axis) displacement. In many commercial instruments,the stylus is connected to a linear variable differential transformer("LVDT") sensor, or to a capacitive plate, for sensing the stylus'vertical displacement. Typically, the stylus includes an elongated barthat is secured with a pair of coaxial pivots, while the other end ofthe stylus is coupled to the Z-axis displacement sensing mechanism, e.g.either a capacitor's plate for a capacitive sensor, or a ferromagneticplunger of the LVDT sensor.

Very sensitive flexure pivot assemblies are commonly used for supportingthe stylus used for classical profilometry. The components of such aflexure pivot assembly are small, delicate, require precision assembly,and therefore are expensive to manufacture. In addition, machining suchstylus assemblies from discrete components tends to make themcomparatively large, and the sensing elements to which they couple arealso relatively large. Therefore, profilometer heads including thestylus are, in general, larger than desirable. Consequently,profilometer heads generally respond slowly to vertical displacements,and the scanning speed at which profilometers operate is limited by theinertia of the profilometer's head. Hence, improving profilometerperformance while concurrently reducing their manufacturing cost andcontact force makes gentler, smaller, lighter, faster and less expensiveprofiling heads very desirable.

The more recently developing field of AFM for measuring a surface'stopography generally uses a very light, micromachined, bendablecantilever probe having a sharp tip for sensing a surface's topology atatomic dimensions. However, systems for detecting minute verticaldisplacement of an AFM's sensing probe, e.g. optical-beam-deflection oroptical interferometry, are, in general, much larger than the cantileveritself. Consequently, it is generally difficult to move an AFM's headassembly as swiftly as desired for high speed scanning. Traditionally,AFM systems circumvented this problem by holding the sensing headassembly stationary while moving the sample along orthogonal X and Yaxes. Although such a system may move small samples easily during AFMscanning, it is generally unsuited for use on large samples, such assemiconductor wafers or magnetic recording disks measuring severalinches in diameter.

Accordingly, not only does AFM necessarily require a physically smallAFM sensing probe, but advancing AFM technology and performance alsomakes a correspondingly small, light weight, and compact sensor fordetecting AFM probe Z-axis displacement desirable. Integration of acompact vertical displacement sensor into an AFM probe would yield asmall, light weight, and compact AFM head having a low mass. Such a lowmass AFM probe would permit very high speed scanning along orthogonalX-axis and Y-axis directions by a small and compact X-axis and Y-axisdrives.

Referring now to FIG. 1, depicted there is a prior art AFM orprofilometer system referred to by the general reference character 20.The system 20 includes a XY axes drive 22 upon which rests a sample 24.The XY axes drive 22 scans the sample 24 laterally with respect to asensing head 26 along a X-axis 32 and a Y-axis 34 that are orthogonal toeach other, or along any other arbitrary axes obtained by compoundmotion along the X-axis 32 and the Y-axis 34. In the instance of an AFM,to provide rapid movement along the axes 32 and 34 the XY axes drive 22may be provided by a piezo electric tube having 4 quadrant electrodes.As the XY axes drive 22 moves the sample 24 laterally, a probe tip orstylus 36 lightly contacts an upper surface 38 of the sample 24 whilemoving up and down vertically parallel to a Z-axis 42 in response to thetopology of the upper surface 38. In the illustration of FIG. 1, theprobe tip or stylus 36 is secured to a distal end of an elongatedcantilever arm 44 extending outward from the sensing head 26. Thesensing head 26, which may if necessary be servoed up and down parallelto the Z-axis 42, senses vertical deflection of the probe tip or stylus36 by the topology of the sample's upper surface 38. A signaltransmitted from the sensing head 26 to some type of signal processingdevice permits recording and/or displaying the topology of the uppersurface 38 as detected by the system 20.

AFM applications of systems such as of the system 20 experiencesubstantial cross coupling among movements along the mutuallyperpendicular axes 32, 34, and 42. Consequently, movement of the sample24 with respect to the AFM sensing head 26, and frequently even themeasurement of such movement, are insufficiently precise for metrologicapplications. Consequently, at present AFM performance may be adequatefor imaging, but not for metrology. The mass of the sample 24 itself(such as an 8 inch diameter semiconductor wafer) impedes high speed,precise movement of the sample 24. Therefore, scanning a massive sample24 swiftly requires holding the sample 24 fixed while scanning thesensing head 26.

FIG. 2 depicts an alternative embodiment, prior art AFM or profilometersystem. Those elements depicted in FIG. 2 that are common to the AFM orprofilometer system depicted in FIG. 1 carry the same reference numeraldistinguished by a prime (') designation. In the system 20' depicted inFIG. 2, the sample 24' rests on a base plate 48 which also supports a XYstage 52. In scanning the sample 24' using the system 20', the XY stage52 moves the sensing head 26' carrying the cantilever arm 44' parallelto the orthogonal X-axis 32' and Y-axis 34', or along any otherarbitrary axes obtained by compound motion along the X-axis 32' and theY-axis 34'.

E. Clayton Teague, et al., in a technical article entitled "Para-flexStage for Microtopographic Mapping" published the January 1988, issue ofthe Review of Scientific Instruments, vol. 59 at pp. 67-73 ("the Teagueet al. article"), reports development of a monolithic, Para-flex XYstage 52, that the article describes as being machined out of metal. Theembodiments of the monolithic plate of such an XY stage 52 is depictedboth in FIG. 3a and 3b. The XY stage 52 depicted in both FIGS. includesan outer base 62 that is fixed with respect to the system 20'. The outerbase 62 is coupled to and supports a Y-axis stage 64 by means of fourstage suspensions 66. Each of the stage suspensions 66 consists of anintermediate bar 68, one end of which is coupled to the outer base 62 bya flexure 72, and another, distal end of the intermediate bar 68 iscoupled to the Y-axis stage 64 by a second symmetrical flexure 72.Similar to the coupling of the outer base 62 to the Y-axis stage 64, theY-axis stage 64 is coupled to and supports a X-axis stage 74 by means offour stage suspensions 66 that are identical to the stage suspensions 66which couple the outer base 62 to the Y-axis stage 64. The stagesuspensions 66 coupling the outer base 62 to the Y-axis stage 64 and thestage suspensions 66 coupling the Y-axis stage 64 to the X-axis stage 74are oriented perpendicular to each other. Consequently, the inner X-axisstage 74 moves substantially perpendicularly to movement of the Y-axisstage 64, with both stages 64 and 74 moving with great accuracy withrespect to the outer base 62. Movement of the stages 64 and 74 withrespect to the outer base 62 is effected by a pair of mutuallyorthogonal stepping-motor-controlled micrometer screw drives, notillustrated in any of the FIGS., which respectively have a pushrodconnection to the Y-axis stage 64 and the X-axis stage 74. The screwdrives extend from outside the outer base 62 through drive apertures 76to respectively contact the Y-axis stage 64 and the X-axis stage 74. TheXY stage 52 depicted in FIG. 3b differs from that depicted in FIG. 3a inthat the stage suspensions 66 coupling the Y-axis stage 64 to the X-axisstage 74 are folded which reduces the space occupied by the XY stage 52.The XY stage 52 reported by C. Teague, et al. provides accurate movementalong mutually perpendicular axes 32' and 34'. However, the XY stage 52depicted in FIGS. 3a and 3b provides no motion amplification.

FIG. 4 depicts the flexure 72 indicated on the XY stage 52 depicted inFIG. 3b. The flexure 72 employs a pair of webs 82 arranged in a W-shapedconfiguration to span between the outer base 62 and the intermediate bar68, between the intermediate bar 68 and the Y-axis stage 64, between theY-axis stage 64 and the intermediate bar 68, and between theintermediate bar 68 and the X-axis stage 74. The flexure 72 depicted inFIG. 4 permits both longitudinal stretching and rotation.

If the XY stage 52 is made by conventional techniques, even a monolithicXY stage 52 such as that disclosed in the Teague et al. article, theresonance frequency is typically a few hundred Hz.Stepping-motor-controlled micrometer screw drives or other forms of pushrods for displacing the XY stage 52 are typically limited to relativelylow frequency operation. Consequently, the XY stage 52 of an AFM canonly be servoed at relatively low speed.

Recent advances in reactive ion etching processes and apparatus foretching silicon permit forming deep vertical structures. For example thenew Alcatel etcher produces etching aspect ratios of 300/1, andtherefore permits etching through wafers several hundred microns thick.Other etchers having similar performance are now available. Sometechniques for wet etching, (such as 100! orientation etching), may alsobe used to fabricate structures having correspondingly high aspectratios. These improved processes permit construction of structures ofmetrologic precision with macro dimensions. This method therefore makesit possible to construct structures of aspect ratios that normally canonly be achieved by EDM (electric discharge machining) of metals. Theseadvances in micromachined silicon fabrication technology permitsexecuting classical designs for the XY stage 52 to provide metrologicquality.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a topographic head inwhich a Z-axis sensor is integrated into the topographic head.

Another object of the present invention is to provide a topographic headfor profilometry having a shape that is defined by photolithography, andthat is made from silicon.

Another object of the present invention is to provide a topographic headfor profilometry with less contact force.

Another object of the present invention is to provide a smallertopographic head for profilometry.

Another object of the present invention is to provide a lightertopographic head for profilometry.

Another object of the present invention is to provide a topographic headadapted for use in AFM having both a Z-axis sensor and tip integratedinto the topographic head.

Another object of the present invention is to provide a topographic headfor use in AFM having very good sensitivity, low contact force, low costand fast response.

Another object of the present invention is to provide a topographic headfor use in AFM that is adapted for "rocking mode operation."

Another object of the present invention is to provide a topographic headhaving simpler topographic head motions.

Another object of the present invention is to provide a topographic headadapted for "rocking mode operation" which decouples the sensing head'sdriving mode from the sensing head's sensing mode.

Another object of the present invention is to provide an inherentlylow-cost topographic head.

Another object of the present invention is to provide a higherperformance topographic head.

Another object of the present invention is to provide a topographic headthat provides a flexible design.

Another object of the present invention is to provide an easilyfabricated topographic head.

Another object of the present invention is to provide an AFM XYZ stagehaving a very fast response.

Another object of the present invention is to provide a metrologicquality XY stage.

Another object of the present invention is to provide an AFM XYZ stagethat is simpler to fabricate.

Briefly, the present invention is a micromachined, topographic head,adapted for use in sensing topography of a surface, that has an outerframe from which torsion bars project inwardly to support a centralpaddle. The torsion bars are aligned along a common axis therebypermitting the central paddle to rotate about the common axis. Theframe, torsion bars and central paddle are all monolithically fabricatedfrom a semiconductor single-crystal silicon wafer. The central paddledefines a rest plane if no external force is applied to the centralpaddle, and is rotatable about the common axis by a force applied to thecentral paddle. The central paddle also includes a tip that projectsoutward from the central paddle distal from the torsion bars, the tipbeing adapted for sensing the topography of a surface. The topographichead also includes drive means for imparting rotary motion to thecentral paddle about the common axis, and a rotational-position sensingmeans for measuring the rotational position of the central paddle aboutthe common axis of the torsion bars.

The present invention also includes a micromachined XYZ stage forsupporting, and carrying for X-axis, Y-axis and Z-axis translation,various different types of scanning microscopy sensors such as thetopographic sensor described above, or an optical near-field, tunneling,or field-emission microscope sensor. The XYZ stage includes an outerstage-base that is adapted to be held fixed with respect to a surface tobe scanned. The outer stage-base is coupled to and supports anintermediate X-axis stage via a plurality of flexures disposed betweenthe outer stage-base and the intermediate X-axis stage. At least one ofthe flexures coupling between the stage-base and the X-axis stage has ashear stress sensor formed therein for sensing stress in that flexure.The intermediate X-axis stage is coupled to and supports an inner Y-axisstage via a plurality of flexures. At least one of the flexures couplingbetween the X-axis stage and the Y-axis stage has a shear stress sensorformed therein for sensing stress in that flexure. The stage-base,X-axis stage, Y-axis stage, and flexures are all monolithicallyfabricated from a semiconductor wafer. A Z-axis stage may also beincluded to provide an integrated XYZ stage.

The preferred Z-axis stage is in many ways similar to the topographichead described above. The preferred Z-axis stage has torsion bars thatproject inwardly from opposing sides of the Y-axis stage. The torsionbars are aligned along a common axis for supporting a Z-axis paddlewithin the Y-axis stage. The torsion bars and Z-axis paddle arepreferably monolithically fabricated from a semiconductor single-crystalsilicon layer of a substrate together with the stage-base, X-axis stage,Y-axis stage, and flexures. The torsion bars support the Z-axis paddlewithin the Y-axis stage for rotation about the torsion bars' commonaxis. An external force is applied to the Z-axis paddle causes theZ-axis paddle to rotate about the common axis to a rotational-positiondisplaced from the Z-axis paddle's rest plane. The Z-axis stage includesa drive means, preferably piezo-electric disks for urging to the Z-axispaddle to rotate about the common axis of the torsion bars. Arotational-position sensing means, preferably integrated into thetorsion bars, measures the rotational-position of the Z-axis paddleabout the common axis of the torsion bars.

The Z-axis stage of the XYZ stage may carry an AFM sensor that adaptsthe XYZ stage for sensing topography of a surface. More specifically,the AFM mead may be the topographic head described above. Accordingly,the AFM sensor carried by the Z-axis stage includes torsion bars thatproject inwardly from opposing sides of an outer frame, and are alignedalong a common axis to support a central paddle within the Y-axis stage,The torsion bars and central paddle are all monolithically fabricatedfrom a semiconductor single-crystal silicon layer of a substrate. Thecentral paddle is supported for rotation about the common axis of thetorsion bars and defines a rest plane if no external force is applied tothe central paddle. A force applied to the central paddle may rotate itaround the common axis of the torsion bars to a rotational-positiondisplaced from the rest plane. The central paddle includes a tip thatprojects outward from an end of the central paddle distal from thetorsion bars. The tip is adapted for juxtaposition with a surface forsensing the topography thereof.

The preceding invention provides integration in silicon of themechanical components and the electrical sensors required by atopographic head. Accordingly, the present invention substantiallyenhances the performance of profilometers and AFMs, and reduces theirsize, while at the same time reducing their cost.

These and other features, objects and advantages will be understood orapparent to those of ordinary skill in the art from the followingdetailed description of the preferred embodiment as illustrated in thevarious drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that depicts one embodiment of a prior art AFM orprofilometer system in which an XY stage moves a sample laterally whilesensing vertical (Z-axis) deflection of a probe tip or stylus;

FIG. 2 is a diagram that depicts an alternative embodiment of a priorart AFM or profilometer system in which a sample is held fixed while anXY stage moves a sensing head laterally while the head senses vertical(Z-axis) deflection of a probe tip or stylus;

FIGS. 3a and 3b are a plan views illustrated alternative embodiments ofa monolithic, Paraflex XY stage adapted for use in AFM of a typereported by C. Teague, et al.;

FIG. 4 is a perspective view depicting a flexure incorporated into themonolithic, Paraflex XY stage illustrated in FIGS. 3a and 3b;

FIG. 5 is a plan view depicting the frame, torsion bar and centralpaddle of a topographic head in accordance with the present invention;

FIG. 5a is a cross-sectional view of the topographic head depicted inFIG. 5 taken along the line 5a--5a;

FIGS. 6a and 6b are plan views depicting alternative embodiments of afour-terminal torsion sensor located on a torsion bar taken along a line6--6 in FIG. 5;

FIG. 6c is plan view, similar to the plan views of FIGS. 6a and 6b, thatdepicts yet another alternative embodiment of the torsion sensor havingonly three-terminals;

FIG. 7 is a cross-sectional view of a topographic head, similar to thecross-sectional view depicted in FIG. 5a, that illustrates physicalcharacteristics of a substrate semiconductor wafer preferably used forfabricating the topographic head;

FIG. 8 is a plan view depicting an alternative embodiment of the centralpaddle depicted in FIG. 5;

FIG. 8a is a cross-sectional view of the central paddle depicted in FIG.8 taken along the line 8a--8a;

FIG. 9 is a plan view depicting the frame, torsion bar and centralpaddle for an AFM topographic head in accordance with the presentinvention;

FIG. 9a is a cross-sectional view of the AFM topographic head depictedin FIG. 9 taken along the line 9a--9a;

FIG. 10 is a force diagram depicting forces applied to a prior art,cantilever oscillating AFM topographic head;

FIG. 11 is a force diagram depicting forces applied to a torsionaloscillating AFM topographic head in accordance with the presentinvention;

FIG. 12 is a cross-sectional view of an alternative embodimenttopographic head similar to that depicted in FIG. 5a adapted forcapacitively sensing topographic head movement with a pair of capacitorplates disposed on the same side of the topographic head;

FIG. 13 is a cross-sectional view of an alternative embodimenttopographic head similar to that depicted in FIG. 5a adapted forcapacitively sensing topographic head movement with a pair of capacitorplates disposed on opposite sides of the topographic head;

FIG. 14 is a cross-sectional view of an alternative embodimenttopographic head similar to that depicted in FIG. 5a adapted foroptically sensing topographic head movement with a light beam reflectedfrom a surface of the topographic head;

FIG. 15 is a plan view of a micromachined XYZ scanning stage inaccordance with the present invention adapted for use in an AFM whichemploys flexures to permit lateral motion of the XY scanning stage withrespect to a sample;

FIGS. 16a and 16b are alternative plan views of a flexure included inthe XY scanning stage in accordance with the present invention takenalong the line 16--16 of FIG. 15;

FIG. 17a is an enlarged plan view of a Z-axis stage included in the XYZscanning stage taken within the line 17--17 of FIG. 15;

FIG. 17b is a cross-sectional view of the Z-axis stage taken along theline 17b--17b in FIG. 17a depicting fabrication of the XYZ scanningstage from bonded silicon wafers;

FIG. 18 is a cross-sectional view of a XY scanning stage, similar to theview of FIG. 17b depicting assembling and bonding together of the XYscanning stage from a stack of silicon wafers each of which has beenpre-processed to form individual XY scanning stages; and

FIG. 19 is a cross-sectional plan view depicting piezo plates arrangedin a face-to-face configuration and secured within a clamshellarrangement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 5 depicts a micromachined topographic head in accordance with thepresent invention that is identified by the general reference character100, and that is adapted for use in sensing topography of a surface. Thetopographic head 100 includes a planar frame 102 from which inwardlyproject a pair of opposing torsion bars 104. The torsion bars 104 arealigned along a common axis 106 and support a central paddle 108 withinthe frame 102. While FIG. 5a depicts the torsion bar 104 as having arectangular cross-section, the torsion bars 104 need not necessarilyhave the aspect ratio depicted there. The cross-section of the torsionbars 104 may, in fact be square, rounded or trapezoidal. The frame 102,torsion bars 104 and central paddle 108 are all monolithicallyfabricated from a semiconductor single-crystal silicon layer of asubstrate wafer. The central paddle 108 is rotatable about the commonaxis 106 of the torsion bars 104. When using a common 100! orientedsilicon wafer, the torsion bars 104 are preferably oriented along the100! crystallographic direction, or the 110! crystallographic direction.The torsion bars 104 may be hardened by conversion of a surface layertherof into silicon carbide or silicon nitride. The physical propertiesof the torsion bars 104 in relationship to the central paddle 108,particularly with respect to vibrational modes, are those described inU.S. patent application Ser. No. 08/139,397 filed by the inventors ofthe present application on Oct. 18, 1993, and now U.S. Pat. No.5,629,790 entitled "Micromachined Torsional Scanner," which patentapplication is hereby incorporated by reference.

With no external force applied to the central paddle 108, a planarsurface 112 of the central paddle 108 defines a rest plane 114illustrated in FIG. 5a. If an external force is applied to the centralpaddle 108, the central paddle 108 will be displaced from the rest plane114 as indicated by a curved arrow 116. The central paddle 108 alsoincludes a tip 118 that projects outward from the central paddle 108distal from the torsion bars 104. As illustrated in FIG. 5a, the tip 118is adapted to be juxtaposed with a surface 122 of a sample 123 forsensing the topography thereof.

The topographic head 100 also preferably includes a small permanentmagnet 124 (or electromagnet) located on the frame 102 which provides amagnetic field 126, indicated by an arrow in FIG. 5a, that is orientedparallel to the rest plane 114. A planar coil 128, having a pair of coilleads 132 that are brought out onto the frame 102 via one of the torsionbars 104, is deposited on the planar surface 112 of the central paddle108. The planar coil 128 may consist of a single turn coil asillustrated in FIG. 5, or it may consist of a multiple turn coil. In thelatter case, an overlap of one turn or a wire bond connection must beprovided in one of the coil leads 132. An electric current flowingthrough the planar coil 128 generates a magnetic field which interactswith the magnetic field 126 from the permanent magnet 124 to apply atorque to the central paddle 108 that urges the central paddle 108 torotate about the common axis 106. Rotation of the central paddle 108with respect to the frame 102 responsive to an electric current throughthe planar coil 128 permits controlling the force which the tip 118applies to the surface 122.

The torsion bar 104 that does not carry the coil leads 132 preferablyhas a torsion sensor 142 formed thereon. The torsion sensor 142 is ofthe type described both in U.S. patent application Ser. No. 08/139,397identified above, and in U.S. Pat. No. 5,488,862 entitled "MonolithicSilicon Rate-Gyro With Integrated Sensors" which issued on Feb. 6, 1996,which was filed by the inventors of the present application, and whichpatent application and issued patent are incorporated herein byreference. As described in the patent application and patent identifiedabove, the torsion sensor 142 preferably has four metallic sensor leads144 which terminate on the frame 102 in individual torsion sensor pads146. The torsion sensor pads 146 permit bonding or soldering to externalleads, not illustrated in any of the FIGS. An electrical signal producedby the torsion sensor 142 permits sensing the rotational-position of thecentral paddle 108 about the common axis 106 of the torsion bars 104with respect to the frame 102.

As described in the patent application and issued patent identifiedabove and as illustrated in greater detail in FIGS. 6a and 6b, thetorsion sensor 142 is preferably a four-terminal piezo sensor. FIG. 6adepicts an embodiment of the torsion sensor 142 in which an electriccurrent flows parallel to the common axis 106. FIG. 6b depicts analternative embodiment of the torsion sensor 142 in which an electriccurrent flows perpendicular to the common axis 106. When using p-type100! silicon material as a substrate for fabricating the torsion bars104, the crystallographic direction of the torsion bar 104 should bealong the 100! axis. When using n-type 100! silicon material as asubstrate for fabricating the torsion bars 104, the crystallographicdirection should be along the 110! axis. As set forth above, either ofthese crystallographic directions are compatible with fabrication of thetorsion bars 104. FIGS. 6a and 6b also illustrate rounded corners 148joining the torsion bars 104 to the frame 102 and to the central paddle108.

To constrain electric current flowing through the torsion sensor 142 tothe planar surface 112 of the torsion bar 104, a sensor region 152 ofthe torsion bar 104 is implanted or diffused with a dopant material. Forexample if torsion bar 104 is fabricated either using p-type or n-typesubstrate material oriented along 100! crystallographic direction, thena p+ dopant is implanted or diffused into the sensor region 152 of theplanar surface 112. While implantation of a p+ dopant material into ann-type substrate produces a junction isolation, in either case theelectric current will be constrained to the planar surface 112. Likewiseif n-type or p-type substrate material is used with the torsion bar 104oriented in the 110! crystallographic direction, an n+ dopant isimplanted or diffused to produce an n-type sensor region 152 for thetorsion sensor 142, either without or with junction isolation. If thetorsion bars 104 are thick with respect to separation between opposingpairs of torsion sensor electrodes 154, in principle implantation ordiffusion may be omitted.

The metallic sensor leads 144 all form ohmic contacts to the implantedor diffused sensor region 152, but are otherwise electrically isolatedfrom the planar surface 112 of the torsion bar 104. During operation ofthe topographic head 100, an electric current is applied to torsionsensor leads 144a and 144b. Torsion stress in the torsion bar 104,illustrated in FIGS. 6a and 6b by a double-headed arrow 156, thatresults from rotation of the central paddle 108 with respect to theframe 102, generates a voltage between torsion sensor leads 144c and144d. The voltage generated between the torsion sensor leads 144c and144d is proportional to the current applied through torsion sensor leads144a and 144b, and to the torsion (shear) stress 156 in the torsion bar104. One advantage of the torsion sensor 142 is that it is insensitiveto linear stresses in the torsion bar 104, such as those caused by theweight of the central paddle 108. However, the torsion bars 104 must beof very high, metrologic quality, and must be stress free. Accordingly,the topographic head 100 is preferably fabricated using SOI for thesubstrate material as described in the patent application and issuedpatent identified above. Alternatively, the topographic head 100 may befabricated from conventional silicon wafers using a timed etch forcontrolling the thickness of the torsion bars 104.

FIG. 6c is an alternative embodiment of the torsion sensor 142 whichsplits one of the current torsion sensor electrodes 154 symmetrically intwo parts 154a and 154b. Twisting the torsion bar 104 induces adifferential change in the electric current flowing through the twohalves of the torsion sensor electrodes 154b and 154c. In the torsionsensor 142 depicted in FIG. 6c, separate voltage sensing torsion sensorelectrodes 154 are not required. All orientations of the torsion sensor142 with respect to the crystallographic directions are otherwiseidentical to the four-terminal torsion sensor 142.

As illustrated in FIG. 7, the preferred substrate for fabricating thetopographic head 100 is a Simox or bonded wafer having an insulatinglayer of silicon dioxide 162 separating single crystal silicon layers164a and 164b. The torsion bars 104 are fabricated from the top singlecrystal silicon layer 164a of the substrate wafer. The torsion bars 104and an upper portion of the central paddle 108 may be fabricated byetching from the top side of a substrate wafer with anisotropic orreactive ion etching. The intermediate silicon dioxide 162 provides aperfect etch stop and uniform thickness for the torsion bars 104.Etching the substrate wafer from the bottom side defines the centralpaddle 108 and the frame 102. Then, the silicon dioxide 162 is etchedaway freeing the central paddle 108 from the frame 102 except for thetorsion bars 104. Alternatively, using a conventional silicon wafer thestructure of the topographic head 100 may be etched first from thefront, suitably protected, and then etched from the back with a timedetch to produce the same configuration as with the Simox or bondedwafer.

Referring to FIG. 5, after etching from the bottom side defines thecentral paddle 108, the frame 102 may completely surround and protectthe central paddle 108. Subsequently, a portion 168 of the surroundingframe 102 may be removed by snapping it off along a groove 162 etchedduring fabrication of the torsion sensor 142. Removing the portion 168yields a U-shaped frame 102 with the torsion bars 104 projecting inwardfrom parallel arms of the frame 102. The central paddle 108 may have itslength oriented along the 100! crystallographic direction of thesubstrate wafer thereby providing vertical walls for the central paddle108. Alternatively, the length of the central paddle 108 may be orientedalong the 110! crystallographic direction thereby producing walls of thecentral paddle 108 having an inclination of 54.7°. Artifacts at 45°resulting from 100! etching are readily accommodated by the structure ofthe topographic head 100.

In general, the voltage generated ΔV is described as ΔV=π₄₄ τV, whereπ₄₄ is the appropriate element of the piezoelectric tensor for n-type orp-type semiconductor material, τ the applied torsion stress, and V thevoltage applied across the torsion sensor leads 144a and 144b. Thevoltage generated across the torsion sensor leads 144c and 144d canapproach 20% of the applied voltage, and since it is typically generatedwithin the low resistance sensor region 152, has very low noise. Analternating current ("AC") may be applied across a pair of torsionsensor pads 146 to the torsion sensor leads 144a and 144b which causesthe electrical signal produced by the torsion sensor 142 on the torsionsensor leads 144c and 144d to become a modulation envelope of theapplied AC thereby removing any direct current ("DC") offset. The outputsignal from the torsion sensor 142 on torsion sensor leads 144c and 144dis typically received by an instrumentation amplifier, not illustratedin any of the FIGS., that provides very good common mode rejection. Anoutput signal produced by the instrumentation amplifier is proportionalto deflection of the central paddle 108 with respect to the frame 102.The voltage generated by the torsion sensor 142 is, for all practicalpurposes, instantaneous, and therefore has very good frequency response.

As illustrated in FIG. 5a, during operation of the topographic head 100the frame 102 typically is tilted slightly with respect to the surface122 to avoid interference with the sample 123. The topographic head 100is lowered until the tip 118 contacts the surface 122, which registersas a change in an output signal from the torsion sensor 142. This changein the output signal produced by the torsion sensor 142 may be regardedas a reference value for scanning the surface 122. As the topographichead 100 traverses across the surface 122, the output signal from thetorsion sensor 142 changes in response to rotation of the central paddle108 with respect to the frame 102 caused variations in the topography ofthe surface 122.

The central paddle 108, including the tip 118, is preferably balancedaround the common axis 106 of the torsion bars 104. Such a configurationfor the topographic head 100 minimizes deformation of the torsion bars104 while also minimizing the output signal from the torsion sensor 142when the torsion bar 104 is disposed in the rest plane 114. However, thewidth and length of the central paddle 108 on either side of the torsionbars 104 need not be identical. For ease of handling, one side 108a ofthe central paddle 108 may be short and stubby, while another side 108bmay be elongated.

FIG. 8 depicts an alternative embodiment of the central paddle 108depicted in FIG. 5. Those elements depicted in FIG. 8 that are common tothe central paddle 108 depicted in FIG. 5 carry the same referencenumeral distinguished by a prime (') designation. As illustrated in FIG.8, the central paddle 108' surrounding a center 166 may be thinnedsignificantly by anisotropic etching along the 110! or 100!crystallographic directions to reduce the mass of the central paddle108, and to thereby improve the dynamic performance of the topographichead 100. Such thinning of the central paddle 108' about the center 166may yield only a thin membrane spanning an outer frame of the centralpaddle 108' with the exception of a central bar 167. Moreover, thematerial surrounding the center 166 may be completely etched awayleaving the central paddle 108' with only a hollow frame-shape and thecentral bar 167. Such a hollow frame structure is stiff, and reduces theload on torsion bars 104. Such profiles for the central paddle 108 areeasily obtained with the SOI structure used in the preferred mode offabrication.

The structure of the topographic head 100 as described thus far isadapted for use either in a profilometer or in an AFM. However, the tip118 or tip 118', which may be attached on either side of the centralpaddle 108 or central paddle 108' as illustrated respectively in FIGS.5a and 8a, will be different. In adapting the topographic head 100 foruse in a profilometer, an anisotropic pit 168 may be etched in thecentral paddle 108 or 108' to receive the tip 118'. The tip 118 or tip118' of a profilometer may be formed from diamond as is common, or anyother suitable material. Alternatively, for an AFM the tip 118 or 118'may be formed on the central paddle 108 or 108' using the methoddescribed in U.S. Pat. No. 5,201,992 filed in the name of Robert Marcuset al. entitled "Method for Making Tapered Microminature SiliconStructures," which patent is incorporated herein by reference.

Capacitive Motion Sensing

FIG. 12 depicts an alternative embodiment topographic head similar tothat depicted in FIG. 5a adapted for capacitively sensing topographichead movement. Those elements depicted in FIG. 12 that are common to thetopographic head 100 depicted in FIG. 5a carry the same referencenumeral distinguished by a triple prime ('") designation. In theembodiment depicted in FIG. 12, a pair of capacitor plates 182 aredisposed on the same side of the central paddle 108'" similar to suchcapacitive sensing plates disclosed in a technical article entitled "TheInterfacial-Force Microscope" by J. E. Houston and T. A. Michalske thatwas published on Mar. 19, 1992 in vol. 356 of Nature at pages 286-287.FIG. 13 depicts another alternative embodiment topographic head similarto that depicted in FIG. 5a that is also adapted for capacitivelysensing head movement. Those elements depicted in FIG. 13 that arecommon to the topographic head 100 depicted in FIG. 5a carry the samereference numeral distinguished by a quadruple prime ('") designation.In the embodiment depicted in FIG. 13, a pair of capacitor plates 184are disposed on opposite sides of the central paddle 108"". Thecapacitor plates 182 or 184 permit sensing motion of central paddle108'" or 108"". If the topographic head 100'" or 100"" operates in arocking or tapping mode, described in greater detail below, anelectrical signal from the torsion sensor 142 may be fed-back to theplanar coil 128 to induce oscillation of the central paddle 108'" or108"" at the frequency of the principal torsional vibrational mode ofthe central paddle 108'" or 108"". For such a rocking or tappingoperating mode, as is known in the art, the capacitor plates 182 or 184permit sensing motion of the central paddle 108'" or 108"".

Optical Motion Sensing

FIG. 14 depicts another alternative embodiment topographic head similarto that depicted in FIG. 5a that is adapted for optically sensingtopographic head movement. Those elements depicted in FIG. 14 that arecommon to the topographic head 100 depicted in FIG. 5a carry the samereference numeral distinguished by a quintuple prime ('"") designation.In the embodiment depicted in FIG. 14, a reflective surface 186 isformed on the planar surface 112'"" of the central paddle 108'"" forreflecting a beam of light 188. As is well known in the art of AFM asillustrated by FIG. 2 of U.S. Pat. No. 5,412,980 filed in the names ofVirgil B. Elings and John A. Gurley ("the '980 patent"), and by atechnical article entitled "Scanning Force Microscope Springs Optimizedfor Optical-Beam Deflection and With Tips Made by Controlled Fracture"by M. G. L. Gustaffson, et al, published Jul. 1, 1994, in the Journal ofApplied Physics, Vol. 76, No. 1, movement of the central paddle 108'""may be sensed by movement of the reflected beam of light 188 across asurface of an optical detector, not illustrated in any of the FIGS.

XYZ Scanning Stage

The plan view of FIG. 15 depicts a preferred embodiment of amicromachined XYZ scanning stage in accordance with the presentinvention, referred to by the general reference character 200, that isadapted for use in an AFM. The XYZ scanning stage 200 includes an outerstage-base 202 that forms a perimeter of the XYZ scanning stage 200, andthat is adapted to be held fixed within the system 20' with respect tothe surface 122 to be scanned. The stage-base 202 is coupled to andsupports an intermediate X-axis stage 204 through a plurality offlexures 206. The flexures 206 are arranged in pairs with one pair beinglocated at the stage-base 202 and the other being located at the X-axisstage 204, the pair of flexures 206 being joined by an intermediate bar208. Use of this structure for the flexures 206 implements the principleof the double compound linear spring described in a technical article byS. T. Smith et al. entitled "Design and Assessment of Monolithic HighPrecision Translation Mechanisms," that was published during 1987 in theJournal of Physics E: Scientific Instruments, vol. 20 at p. 977. Animportant characteristic of this arrangement for supporting the X-axisstage 204 with respect to the stage-base 202 is that there is there isno stretching of the flexures 206.

In the preferred embodiment of the XYZ scanning stage 200 depicted inFIG. 15, eight pairs of flexures 206, each coupled by the intermediatebar 208, join the stage-base 202 to the X-axis stage 204 to permitmotion of the X-axis stage 204 laterally from side-to-side with respectto the stage-base 202. The X-axis stage 204 encircles, is coupled to andsupports an inner Y-axis stage 212 by an arrangement of flexures 214 andintermediate bars 216 similar to that by which the stage-base 202supports the X-axis stage 204. The flexures 214 and the intermediatebars 216 permit the Y-axis stage 212 to move laterally from side-to-sidewith respect to the X-axis stage 204. Thus compound lateral translationof the stage-base 202 with respect to the X-axis stage 204 and of theY-axis stage 212 with respect to the X-axis stage 204 permitsindependent movement of the Y-axis stage 212 along mutuallyperpendiculary X and Y axes. The entire XYZ scanning stage 200,including the stage-base 202, the X-axis stage 204, the flexures 206,the intermediate bars 208, the Y-axis stage 212, the flexures 214 andthe intermediate bars 216 are monolithically fabricated from asemiconductor single-crystal silicon layer of a substrate.

At least one of the flexures 206 and at least one of the flexures 214includes a shear stress sensor 222 for sensing stress respectively inthe flexure 206 or 214. FIGS. 16a and 16b are plan views depicting boththe flexure 206 or the flexure 214 taken along the lines 16--16 in FIG.15. The stress sensor 222 depicted in FIGS. 16a and 16b may be used tomeasure deflection of the flexures 206 and 214, and hence the stagedeflection respectively along the X or Y axis. Even if longitudinalstresses appear in the flexures 206 and/or 214, the stress sensor 222depicted in FIG. 16a is insensitive to such stresses, and properlyindicates deflection along either the X or the Y axis. Since shearforces are greatest near the center of the flexures 206 and 214, thestress sensor 222, which in the illustration of FIG. 16a has aconfiguration similar to the torsion sensor 142 depicted in FIG. 6a,should be located on the flexure 206 or 214 as depicted in FIG. 16a.

In the stress sensor 222 depicted in FIG. 6a, a pair of shear-sensorcurrent-leads 224 provide an electric current to the stress sensor 222,and a pair of shear-sensor sensing-leads 226 sense a voltage induced byshear stress in the flexure 206 or 214. The voltage present on theshear-sensor sensing-leads 226 is proportional to deflection of theflexure 206 or 214. The center axis 228 of the flexure 206 or 214 shouldbe oriented in the 100! crystallographic direction for p-type silicon,and in the 110! crystallographic direction for n-type silicon. However,the stress sensor 222 may be fabricated either with the orientationdepicted in FIG. 16a, or in an orientation rotated at 900 from thatdepicted in FIG. 16a. The stress sensor 222 requires no particularcurrent isolation such as that required for the torsion sensor 142 sincethe shear stress is the same throughout the thickness of the flexure 206or 214. Since the shear stress is greatest in the center of the flexure206 or 214, current through the stress sensor 222 flows preferably alongthe center axis 228.

An alternative bending stress sensor 222 depicted in FIG. 16b employs apair of piezo resistors 232 disposed symmetrically on opposite sides ofthe center axis 228. In such a piezo resistor implementation, bending ofthe flexure 206 or 214 compresses one piezo resistor 232 whilestretching the other piezo resistor 232. However, piezo resistors 232respond both to bending stresses of the flexure 206 or 214 and totensile or compressive stress along the center axis 228, to which thestress sensor 222 depicted in FIG. 16a is insensitive. Use of the piezoresistors 232 in a differential mode reduces sensitivity of the piezoresistors 232 to tensile or compressive stress along the center axis228, while responding preferentially to bending stress in the flexure206 or 214. The stress sensor 222 depicted in FIG. 16b may also includeadditional piezo resistors 234, preferably removed from the area ofbending stresses and/or oriented to be insensitive to bending stress,e.g. perpendicular to and symmetrically about the center axis 228. Thepiezo resistors 234 may be electrically incorporated into a resistancebridge together with the piezo resistors 232 to provide temperaturecompensation for the stress sensor 222.

The Y-axis stage 212 may support and translate along X and Y axesvarious different types of scanning sensors such an optical near-fieldmicroscope, a tunneling microscope, a field emission microscope, or atopographic head 100 such as that described above. The embodiment of theXYZ scanning stage 200 depicted in FIG. 15 illustrates, in greaterdetail in FIGS. 17a and 17b, the XYZ scanning stage 200 supporting atopographic head 100. As depicted in those FIGS., the Y-axis stage 212includes a Z-axis stage 238 having a pair of torsion bars 242 thatproject inwardly from opposing sides of the Y-axis stage 212. Thetorsion bars 242 are aligned along a common axis 244 for supporting aZ-axis paddle 246 within the Y-axis stage 212. The torsion bars 242 andthe Z-axis paddle 246 are monolithically fabricated from a semiconductorsingle-crystal silicon layer of a substrate together with the stage-base202, X-axis stage 204, Y-axis stage 212, the flexures 206 and 214, andthe intermediate bars 208 and 216. Similar to the central paddle 108described above, the Z-axis paddle 246, which is supported within theY-axis stage 212 for rotation about the common axis 244 of the torsionbars 242, defines a rest plane if no external force is applied to theZ-axis paddle 246. The Z-axis paddle 246 is rotatable about the commonaxis 244 of the torsion bars 242 to a rotational-position displaced fromthe rest plane by a force applied to the Z-axis paddle 246. The Z-axispaddle 246 may carry a topographic head 100 such as that described aboveand below adapted for AFM that projects outward from the Z-axis paddle246 distal from the torsion bars 242. One of the torsion bars 242 of theZ-axis stage 238 includes a torsion sensor 249 fabricated identically tothe torsion sensor 142 described above, and that has a structure androtational sensing function identical to the torsion sensor 142. Theelectrical signal produced by the torsion sensor 249 measures Z-axismotion of the Z-axis paddle 246 with respect to the Y-axis stage 212.

In fabricating the XYZ scanning stage 200, wafers may be etched fromboth sides with mirrored image masks (carefully aligned) to increase thedepth of the flexures 206 and 214, and their height to width aspectratio. Bonded wafers may be used as substrates for fabricating the XYZscanning stage 200. FIG. 17b illustrates the XYZ scanning stage 200fabricated in this manner in which the silicon wafer consists of 2bonded wafers 252a and 252b. The bonded wafers 252a and 252b are joinedthrough an oxide layer 254 between them which acts as an etch stop. Theuse of bonded wafers 252a and 252b allows doubling the aspect ratio ofthe flexures 206 and 214 by etching from both sides either with wetetching or reactive ion etching. If the wall angle of the flexures 206and 214 differs from 90°, then this procedure provides a symmetric shapefor the flexures 206 and 214 which ensures distortion free lateraltranslation of the Y-axis stage 212 with respect to the stage-base 202.

Even higher aspect ratios may be achieved for the flexures 206 and 214by identically etching several semiconductor wafers 262, illustrated inFIG. 18, that include lithographically fabricated alignment holes. Afterthe semiconductor wafers 262 have been fabricated, they may be stackedone on top of the other as depicted in FIG. 18, preferably in pairs oftwo wafers facing back-to-back. The XYZ scanning stages 200 in each ofthe semiconductor wafers 262 may be bonded or glued together. If sidewalls of the flexures 206 and 214 slope as generally occurs if thesemiconductor wafers 262 are prepared using anisotropic wet etching,then the etch direction for each pair of semiconductor wafers 262preferably alternates so the overall stack is symmetric. Only one of thesemiconductor wafers 262, i.e. an outer wafer 262, need include thetorsion bars 242, Z-axis paddle 246 and the stress sensor 222.

In principal the X-axis stage 204 and the Y-axis stage 212 could betranslated laterally with respect to the stage-base 202 by a pair ofmutually orthogonal stepping-motor-controlled micrometer screw drivessuch as those described in the Teague et al. article, or by another typeof push-rod mechanism. However, translation of the X-axis stage 204 andthe Y-axis stage 212 is preferably effected using thin piezo electrictransducers 272 illustrated in FIG. 15. One pair of such piezo electrictransducers 272 is interposed between the stage-base 202 and the X-axisstage 204 on opposite sides of the X-axis stage 204. Similarly, a secondpair of such piezo electric transducers 272 is interposed between theX-axis stage 204 and the Y-axis stage 212 on opposite sides of theY-axis stage 212. The piezo electric transducers 272 may be located inpockets created in the stage-base 202 and in the stages 204 and 212, andmay be operated in tandem from either side if so desired. The piezoelectric transducers 272 have a very low mass and inertia whiledisplacing the stages 204 and 212 sufficiently for AFM operation.Arranged in this way, the X-axis stage 204 must carry the piezo electrictransducers 272 for displacing the Y-axis stage 212. However, thesepiezo electric transducers 272 are light, thus their mass does notsignificantly degrade the performance of the XYZ scanning stage 200.

The piezo electric transducers 272 may be provided either by thin piezoelectric unimorph or bimorph disk or strip shaped plates 274 operatingin the doming mode, preferably used in tandem. The piezo electrictransducers 272 (either single or dual) consist preferably, asillustrated in FIG. 19, of 2 piezo plates 274, positioned face-to-face.The plates 274 may be fabricated from a thin circular disk ofstress-biased lead lanthanum zirconia titanate ("PLZT") material. Thismaterial is manufactured by Aura Ceramics and sold under the "Rainbow"product designation. This PLZT unimorph provides a monolithic structureone side of which is a layer of conventional PLZT material. The otherside of the PLZT unimorph is a compositionally reduced layer formed bychemically reducing the oxides in the native PLZT material to produce aconductive cermet layer. The conductive cermet layer typically comprisesabout 30% of the total disk thickness. Removing of the oxide from oneside of the unimorph shrinks the conductive cermet layer which bends thewhole disk and puts the PLZT layer under compression. The PLZT layer istherefore convex while the conductive cermet layer is concave.

Regardless of the particular material system used for the plates 274,applying a voltage across the plates 274 causes them either to increaseor decrease their curvature. If the piezo electric transducer 272 on oneside of a stage 204 or 212 increases the curvature of the plates 274while the plates 274 in the piezo electric transducer 272 on the otherside decreases its curvature the stage will move laterally with respectto the surrounding stage-base 202 or X-axis stage 204.

The plates 274 may preferentially be put in a clamshell arrangement asillustrated in FIG. 19. The plates 274 are surrounded by metal clamps276, which may have an interior surface shaped to reduce stress byconforming to the curvature of the plates 274. The two metal clamps 276are free to rotate with respect to each other, and are held together bya small spring or a hinge clasp 278. The metal clamps 276, which projectupward above the upper surface of the XYZ scanning stage 200, includejaws 282 that contact adjacent edges of the stage-base 202 and theX-axis stage 204, or of the X-axis stage 204 and the Y-axis stage 212.Preferably the jaws 282 may be coated with plastic. The metal clamps 276may include lips 284 for gluing the piezo electric transducers 272 inplace to the stages 204 and 212. The plates 274 are lapped to athickness which matches the space between adjacent edges of thestage-base 202 and the X-axis stage 204, or of the X-axis stage 204 andthe Y-axis stage 212; the clamshell is then compressed and insertedbetween the stage-base 202 and the X-axis stage 204, or between theX-axis stage 204 and the Y-axis stage 212. The preload on the piezoelectric transducers 272 must be carefully controlled. The maximumcontraction of the plates 274 in response to an applied voltage must besmaller than the preload compression, or the plates 274 will get loosewith consequent loss of control over lateral movement of the Y-axisstage 212.

While as illustrated in FIG. 15 the preferred embodiment of the XYZscanning stage 200 preferably uses a pair of piezo electric transducers272 for displacing the Y-axis stage 212 along each of the X and Y axes.Such a dual arrangement of plates 274 on opposite sides of the stages204 and 212 advantageously provides transducer preload, withoutpronounced deflection of the stage. However, a XYZ scanning stage 200 inaccordance with the present invention need only use a single piezoelectric transducer 272 for effecting movement along the X axis or alongthe Y axis. In such a XYZ scanning stage 200 having only a single piezoelectric transducer 272 per axis, each stage 204 and 212 must bepreloaded against the piezo electric transducer 272 either by forcegenerated within the flexures 206 and 214, or by a spring interposedbetween the stage-base 202 and the X-axis stage 204, and between theX-axis stage 204 and the Y-axis stage 212. These piezo electrictransducers 272 can be inserted from the back of the XYZ scanning stage200 so that the front is unencumbered and may be very close to theobject to be scanned, if required.

To apply a force urging the topographic head 100 carried by the Z-axispaddle 246 toward a surface to be scanned, as illustrated in FIGS. 17aand 17b the XYZ scanning stage 200 includes a disk-shaped bimorph,unimorph or a Rainbow type, stress-biased PLZT piezo transducer 292 thatcontacts the Z-axis paddle 246. Upon application of a voltage to thepiezo transducer 292, it deflects the cantilevered Z-axis paddle 246,thereby providing high frequency vertical motion along the Z-axis. TheXYZ scanning stage 200 carrying the topographic head 100 may be used tomeasure the topography of the surface 122 in either of two differentways. In one mode of operation that may be identified as a constantforce measurement, the electrical signal applied to the piezo transducer292 maintains the signal from the the torsion sensor 142 included in thetopographic head 100 at a constant value thereby causing the tip 118 ofthe topographic head 100 to apply a constant force to the surface 122.In this mode of operation, the signal from the torsion sensor 249indicates the topography of the surface 122. In an alternative mode ofoperation the electrical signal applied to the piezo transducer 292holds the topographic head 100 at a fixed location and the signal fromthe torsion sensor 142 indicates the topography of the surface 122.

Profilometer Head 100

The preceding description of the topographic head 100 is generic both tothe topographic head 100 of a profilometer or of an AFM 20 or 20'.Adapting the topographic head 100 for use in a profilometer requiresfabricating the topographic head 100 with a length of several mm for thecentral paddle 108 orthogonal to the common axis 106, a width ofapproximately 1-2 mm for the central paddle 108 parallel to the commonaxis 106, a thicknesses of tens of microns for the torsion bars 104, awidth on the order of tens to hundreds of microns for the torsion bars104, and lengths from a fraction of a mm to several mm for the torsionbars 104. A large variety of dimensions are possible, depending on theperformance characteristics desired for the topographic head 100. Atopographic head 100 having a length and width for the central paddle108 of 9 mm and 1 mm respectively, and having a length, width andthickness for the torsion bars 104 of 1000, 600 and 20 micronsrespectively, has a principal torsional vibrational mode for the centralpaddle 108 about the common axis 106 of approximately 360 Hz, with thenext higher vibrational mode at a frequency of approximately 2,366 Hz. A10 A° displacement of the tip 118 for such a topographic head 100readily produces a measurable signal from the torsion sensor 142. Muchlarger signals can be obtained from the torsion sensor 142 if thetorsion bars 104 are made stiffer which, however, correspondinglyincreases the uncompensated force which the tip 118 applies to thesurface 122.

AFM Head 100

An AFM topographic head 100, illustrated in FIGS. 9 and 9a, can also befabricated in much the same way as the profilometer topographic head 100using a SOI silicon wafer substrate, but the dimensions of the AFMtopographic head are substantially different. Those elements depicted inFIGS. 9 and 9a depicting an AFM topographic head that are common to thetopographic head 100 depicted in FIG. 5 carry the same reference numeraldistinguished by a double prime (""") designation. For an AFMtopographic head 100", the central paddle 108" typically has the samethickness as the torsion bars 104". The frame 102", central paddle 108",torsion bars 104", tip 118", torsion sensor 142", planar coil 128", andpermanent magnet 124" are identical to the corresponding item for aprofilometer except for being smaller in size. The torsion sensor 142again produces an output signal representative of the topology of thesurface 122" by recording rotation of central paddle 108" with respectto the frame 102" as the tip 118" traverses the surface 122". The tip118" is now typically integrated into the central paddle 108" asillustrated in FIG. 9a, and is generally also made of silicon. Thecentral paddle 108" now has the same thickness as the torsion bars 104because AFM requires a much higher frequency response. The permanentmagnet 124 may now be fixed on the other side of the frame 102" so asnot to touch the substrate, but can also be much thinner. Typicaldimension for an AFM topographic head 100" are: 100 and 200 microns forthe length and width of the central paddle 108"; the torsion bars 104are 150 microns long, 20 microns wide, 10 microns thick. Such atopographic head 100" has a principal torsional vibrational mode atapproximately 275 kHz with the next higher vibrational mode atapproximately 360 kHz, and a contact force of 2.3×10-8 Nt per A°. Thestress generated in the torsion bars 104 is 4700 dynes/cm² for 1 A°deflection which produces a signal adequate for detection.

The topographic head 100" disclosed herein is extremely well suited foruse in rocking or "tapping" mode AFM. In the tapping mode AFM, such asthat described in the '980 patent, the tip 118" is first made tooscillate with a controlled small amplitude while not contacting thesurface 122, and is then brought into contact with the surface 122thereby reducing the amplitude of the oscillation. A force diagram inFIG. 10 schematically illustrates both a cantilever vertical tappingmode 172, indicated by a double headed arrow in FIG. 10, and a verticalservo motion 174 of the AFM sensing head, also illustrated by a doubleheaded arrow in FIG. 10. In the force diagram of FIG. 10, the cantileververtical tapping mode 172 and the vertical servo motion 174 are crosscoupled. Conversely, as illustrated in the force diagram of FIG. 11, thecentral paddle 108" described herein does not couple the vertical servomotion 174 to the signal from the torsion sensor 142. Consequently, asindicated in FIG. 10 there is no cross-coupling between the verticalservo motion 174 and the tapping or rocking of the central paddle 108".The motion of the balanced central paddle 108" is not affected byvertical servo displacements, while the cantilever illustrated by FIG.10 is affected by vertical servo displacements which is undesirable.

Typical dimensions for an AFM rocking mode topographic head 100" are:central paddle 108" length 500 microns, thickness 20 microns, width 200microns; torsion bar 104 length 100 microns, width 50 microns andthickness 20 microns. The resonance frequency for the principaltorsional vibrational mode of the central paddle 108" is thenapproximately 250 kHz. In the rocking mode of AFM operation, currentsupplied to the planar coil 128 keeps the amplitude of the signal fromthe torsion sensor 142" constant though use of a feedback circuit tosupply current to the planar coil 128", after suitable amplification.The central paddle 108" is made self-oscillating at its principaltorsional vibrational frequency by feeding back the signal from thetorsion sensor 142" to the planar coil 128". Preferably, thisalternating current passing through planar coil 128 causes the centralpaddle 108" to oscillate at its principal torsional vibrational mode,with a small amplitude of oscillation (several tens to hundreds of A°)at the tip 118". As described previously, the entire topographic head100" is then servoed vertically with the sensing head 26' to keep theamplitude of torsional oscillation constant as the tip 118" contacts thesurface 122". Accordingly, in such a rocking mode AFM the vertical servosignal becomes the output signal that represents the topology of thesurface 122". The small size of the topographic head 100", and its highdegree of integration, adapts the topographic head 100" for very fastscanning. For the rocking mode AFM topographic head 100' describedabove, a 1 A° change in oscillation amplitude can be observed with a 22dB S/N ratio.

Other Uses for Topographic head 100

The profilometer and AFT configuration of the topographic head 100described above can also be used as a micro-indentor for evaluating theproperties of various thin coatings. To use the topographic head 100 asa micro-indentor, the tip 118 is positioned in contact with thesubstrate, a calibrated pulse of current is sent through planar coil128, and impact force of the tip 118 indents surface 122 of the sample123. The indented area of the sample 123 may then be scanned to measurethe indentation formed in the surface surface 122.

XY Scanning Stage 200

A XYZ scanning stage 200 in accordance with the present invention may befabricated having an outer dimension of 35×27 mm for the stage-base 202while the inner Y-axis stage 212 is 5×5 mm. Each pair of flexures 214and intermediate bars 216 is 3 mm long, 600 micron wide, 800 micronthick, and the thinnest part of the flexure 214 is 100 microns wide. Thelowest resonance frequencies in such a system are all above 3000 Hz.Displacements of the Y-axis stage 212 along the X and Y axes maytypically be 50 microns, and the Z axis displacement a few microns.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is purely illustrative and is not to be interpreted aslimiting. Consequently, without departing from the spirit and scope ofthe invention, various alterations, modifications, and/or alternativeapplications of the invention will, no doubt, be suggested to thoseskilled in the art after having read the preceding disclosure.Accordingly, it is intended that the following claims be interpreted asencompassing all alterations, modifications, or alternative applicationsas fall within the true spirit and scope of the invention.

What is claimed is:
 1. A micromachined XY scanning stage comprising:anouter stage-base that is adapted to be held fixed with respect to asurface to be scanned; an intermediate X-axis stage that is coupled toand supported from the stage-base by a plurality of flexures, at leastone of the flexures coupling between said stage-base and said X-axisstage having a shear stress sensor formed therein for sensing stress inthat flexure; an inner Y-axis stage that is coupled to and supportedfrom the X-axis stage by a plurality of flexures, at least one of theflexures coupling between said X-axis stage and said Y-axis stage havinga shear stress sensor formed therein for sensing stress in that flexure;said stage-base, X-axis stage, Y-axis stage, and flexures all beingmonolithically fabricated from a semiconductor single-crystal siliconlayer of a substrate; and sensing means supported by, and carried forX-axis and Y-axis translation by, said X-axis stage and Y-axis stage. 2.The XY scanning stage of claim 1 wherein said sensing means adapts theXY scanning stage for sensing topography of a surface, said sensingmeans including:a Z-axis stage having torsion bars that project inwardlyfrom opposing sides of said Y-axis stage and are aligned along a commonaxis for supporting a Z-axis paddle within said Y-axis stage; saidtorsion bars and Z-axis paddle being monolithically fabricated from asemiconductor single-crystal silicon layer of a substrate together withsaid stage-base, X-axis stage, Y-axis stage, and flexures; said Z-axispaddle being supported within the Y-axis stage for rotation about thecommon axis of the torsion bars, defining a rest plane if no externalforce is applied to said Z-axis paddle, and being rotatable about thecommon axis of said torsion bars to a rotational-position displaced fromthe rest plane by a force applied to said Z-axis paddle; said Z-axispaddle being adapted for carrying a scanning sensor; drive means forurging to said Z-axis paddle to rotate about the common axis of saidtorsion bars; and rotational-position sensing means for measuring therotational-position of said Z-axis paddle about the common axis of saidtorsion bars.
 3. The XY scanning stage of claim 2 wherein the scanningsensor carried by said Z-axis stage is a micromachined topographic headadapted for use in sensing topography of a surface, the topographic headincluding:a frame from which inwardly project opposing torsion bars thatare aligned along a common axis and that support a central paddle withinsaid frame; said frame, torsion bars and central paddle all beingmonolithically fabricated from a semiconductor single-crystal siliconlayer of a substrate; said central paddle being supported within theframe for rotation about the common axis of the torsion bars, having acenter, defining a rest plane if no external force is applied to saidcentral paddle, and being rotatable about the common axis of saidtorsion bars to a rotational-position displaced from the rest plane by aforce applied to said central paddle; said central paddle including atip that projects outward from said central paddle distal from saidtorsion bars, the tip being adapted for juxtaposition with a surface forsensing the topography thereof; drive means for urging to said centralpaddle to rotate about the common axis of said torsion bars; androtational-position sensing means for measuring the rotational-positionof said central paddle about the common axis of said torsion bars. 4.The XY scanning stage of claim 2 wherein said drive means is a laminatedmetal unimorph that is coupled to said Z-axis paddle.
 5. The XY scanningstage of claim 2 wherein said drive means is a bimorph that is coupledto said Z-axis paddle.
 6. The XY scanning stage of claim 2 wherein saiddrive means is formed from stress-biased PLZT material of a Rainbow typeof ceramic which has been processed so one side surface thereof has beencompositionally reduced to obtain a material having a cermetcomposition, whereby the drive means constitutes a monolithic unimorph,the unimorph being coupled to said Z-axis paddle.
 7. The XY scanningstage of claim 1 wherein the shear stress sensor includes a piezosensor.
 8. The XY scanning stage of claim 1 wherein the shear stresssensor includes a piezo resistor.
 9. The XY scanning stage of claim 1further comprising X-axis drive means.
 10. The XY scanning stage ofclaim 9 wherein said X-axis drive means is interposed between said outerstage-base and said intermediate X-axis stage.
 11. The XY scanning stageof claim 9 wherein said X-axis drive means is a piezo transducerinterposed between said outer stage-base and said intermediate X-axisstage.
 12. The XY scanning stage of claim 11 wherein said piezotransducer is a laminated metal unimorph.
 13. The XY scanning stage ofclaim 11 wherein said piezo transducer is a bimorph.
 14. The XY scanningstage of claim 11 wherein said piezo transducer is formed fromstress-biased PI, ZT material of a Rainbow type of ceramic which hasbeen processed so one side surface thereof has been compositionallyreduced to obtain a material having a cermet composition, whereby thepiezo transducer constitutes a monolithic unimorph.
 15. The XY scanningstage of claim 1 further comprising Y-axis drive means.
 16. The XYscanning stage of claim 15 wherein said Y-axis drive means is interposedbetween said intermediate X-axis stage and said inner Y-axis stage. 17.The XY scanning stage of claim 15 wherein said Y-axis drive means is apiezo transducer interposed between said intermediate X-axis stage andsaid inner Y-axis stage.
 18. The XY scanning stage of claim 17 whereinsaid piezo transducer is a laminated metal unimorph.
 19. The XY scanningstage of claim 17 wherein said piezo transducer is a bimorph.
 20. The XYscanning stage of claim 17 wherein said piezo transducer is formed fromstress-biased PLZT material of a Rainbow type of ceramic which has beenprocessed so one side surface thereof has been compositionally reducedto obtain a material having a cermet composition, whereby the piezotransducer constitutes a monolithic unimorph.